As future generations of electronic devices advance in complexity and decrease in size, a growing need exists for a dielectric material more effective than SiO2. Increased demands on ultra-large scale integrated (ULSI) circuits have required that the SiO2 that forms the gate oxide of field-effect transistors be made thinner and thinner. Eventually, however, the SiO2 layers will be required to be so thin that electron tunneling will make current leakage unacceptably high for low-power devices.
Amorphous hafnium oxide (HfO2), also known as hafnia, has been identified as a promising candidate to replace SiO2 as a gate dielectric. Hafnium oxide, having a relatively high bulk dielectric constant (k=25), would allow gate oxides to be physically thicker (for a given capacitance), which could significantly reduce tunneling. Hafnium oxide also exhibits a large band gap (approximately 5.7 eV) and a band offset (greater than 1 eV) with substrates such as silicon. Further, the diffusion of hafnium atoms into substrates such as silicon, particularly during or after post-deposition anneals, has proven to be negligible.
However, amorphous hafnium oxide tends to crystallize at relatively low temperatures (approximately 400° C.) to form monoclinic and/or tetragonal crystallites. Polycrystalline hafnium oxide facilitates unwanted metal or impurity diffusion through grain boundaries and degrades gate stack performance. Further, the surface of polycrystalline hafnium oxide is rough, with varied crystal orientation, making it difficult to control the workfunction of the overlying gate metal that is dependent on the surface crystal orientation of the polycrystalline hafnium oxide layer.
Accordingly, it is desirable to provide a semiconductor structure that comprises a modified hafnium oxide layer that exhibits relatively lower leakage current density, higher crystallization onset temperatures and stability in the amorphous phase. In addition, it is desirable to provide a method for fabricating a semiconductor structure that comprises a modified hafnium oxide layer that exhibits relatively higher crystallization onset temperatures and stability in the amorphous phase. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.